The present invention relates to a gas turbine having an active clearance control system to ensure rotating blades do not rub against their seals in the stationary casing of the gas turbine.
In a gas turbine, combustion gases flow through an annular gas passage between a casing for the turbine and a rotor. Rotating blades, e.g., buckets, extend radially out from the rotor and into the annular gas passage. The hot combustion gases flowing over the blades rotate the turbine and thereby drive the gas turbine.
To maximize the conversion of energy from the hot combustion gasses to mechanical energy, it is desired to have all of the combustion gases flow through the blades of the turbine and avoid leakage of the combustion gases into the casing of the turbine or through gaps between the tips of the turbine blades and the inner surfaces of the turbine casing. The casings may include annular seal surfaces that extend around the tips of the turbine buckets. Minimizing the gap between the annular seal surfaces and the tips of the blades reduces the leakage of combustion gases and increases the thermodynamic efficiency of the gas turbine.
Because the gap is small between the seals in the casings and the tips of the blades, there is a risk that the tips of the blades will impact, e.g., rub, against the seals. This risk of rubbing is increased as the gas turbine is heated during startup and cools during shutdown. The thermal expansion rates of the casing and seals tend to be different than the thermal expansion rates of the turbine rotor, which includes the blades. The differential in the thermal expansion between the casing and the rotor causes variations in the clearance between the tips of the blades and the seals, as the gas turbine transitions during startup, shutdown and other load changes. If the clearance becomes too small, the blade tips may impact, e.g., rub, against the seals of the casing. Rubs can damage the gas turbines, such as by breaking or otherwise damaging the blades and seals. Accordingly, there is a long felt need for improved methods and apparatuses to actively control the blade tip clearances in a gas turbine.